This invention relates generally to X-ray tubes and more particularly concerns adding a non-glass insert to the vacuum tube of an X-ray tube to reduce the noise produced by the X-ray tube.
The use of X-ray tubes for medical diagnostics is quite common. Unfortunately, an X-ray tube operating at a steady state condition generates significant levels of high frequency noise. The sound pressure may vary from one X-ray tube design to another, but is often in the range of 65-75 decibels and can be even higher. This level is typically 10-20 dB higher than the background noise level in a common medical environment. Thus X-ray tube noise is a problem because the current noise levels are not only a general annoyance to patients, doctors and medical personnel but also makes patient-doctor communications difficult.
A conventional X-ray tube comprises a vacuum tube mounted within an outer casing. The remaining interior space of the casing is filled with oil. The oil dissipates heat generated in the vacuum tube and serves as a dielectric or electrical insulator. The vacuum tube includes a target which is bombarded with electrons emitted from an electron emitter. The electrons cause the target to emit X-ray radiation. In order to prevent its rapid deterioration, the target is rotated at approximately 10,000 RPM. To accomplish this rotation, the target is mounted to a rotatable shaft which is coupled to the vacuum tube via bearings. The rotation of the target generates vibration which is transmitted through the bearings to the vacuum tube. Since the oil is essentially incompressible, the vibration is readily transmitted from the vacuum tube to the outer casing with little attenuation. When the vibration energy is received by the casing, the casing radiates sound to the ambient. The casing is very thick and has a non-uniform spatial distribution so that the chances to reduce sound radiation from the casing are very limited. Although the vibration energy transmitted to the casing could be reduced by lowering the rotational speed of the target, this would severely shorten the life expectancy of the target unless the X-ray power was lowered in accordance with the lowered rotational speed. However, lowering the X-ray power would sacrifice the quality of the image.
One means for reducing X-ray tube noise without shortening target life expectancy or sacrificing image quality is described in U.S. Pat. No. 4,935,948, issued Jun. 19, 1990 to Jeung T. Kim. U.S. Pat. No. 4,935,948 discloses attaching a ring mass on or near the bearing shroud which physically connects the rotor bearings to the vacuum tube. The ring mass dissipates the vibrational energy which would otherwise be transmitted from the rotor bearing to the vacuum tube and on to the casing. Thus, the addition of the ring mass reduces the total noise produced by the X-ray tube. The use of a ring mass, which weighs about two pounds or more, significantly increases the overall weight of the X-ray tube. Furthermore, if made of an electrically conductive material, the ring mass creates an electrical flashover problem because it is located near the very high voltage region within the X-ray tube.